The Spin Lattice Relaxation of the Nuclear Dipolar Energy in Some Organic Crystals with Slow Molecular Motions

The relaxation of nuclear dipolar energy to the lattice has been measured in three different organic solids (benzene, cyclohexane and hexamethylbenzene) as a function of temperature. In the cases of C6H8 and C 6 (CH 3 ) 6 very slow motions associated with rather high activation energy were detected near the melting point. They are thought to be thermally activated rotations of the molecules about axes other than the hexad axis. In the case of cyclohexane the activation energy for the process of molecular diffusion was determined directly from the experimental results without the need of a model for vacancy diffusion. A maximum in dipolar relaxation rate was always observed for correlation times of the order of the rigid lattice line width i. e. in the temperature region where the lines narrow.